1. Field of the Invention
The present invention relates to a thin-film electroluminescence (EL) display device used, for example, as an area light source for back lighting an instrument or the like.
2. Description of Related Art
Thin-film EL display devices, utilizing the emission of light by phosphorescent substances under the influence of an applied electric field, have been attracting attention as components for forming self luminous flat panel displays.
FIG. 1 is a schematic diagram showing a typical cross sectional structure of a thin-film EL display device 10 of prior art.
The thin-film EL display device 10 comprises a first electrode 2 of an optically transparent ITO (indium tin oxide) film, a first insulating layer 3 made of tantalum pentoxide (Ta.sub.2 O.sub.5) or the like, a light-emitting layer 4, a second insulating layer 5, and a second electrode 6 of an ITO film, which are successively deposited one on top of another on an insulating glass substrate 1.
The ITO film, a transparent conductive film made of indium oxide (In.sub.2 O.sub.3) doped with tin (Sn), has been widely used for a transparent electrode because of its low resistivity.
The light-emitting layer 4 is made, for example, of zinc sulfide as the base material, with additions of manganese (Mn) or terbium trifluoride (TbF.sub.3) as the luminescence center. The luminescent color of the thin-film EL display device is determined by the kind of additive in the zinc sulfide, the luminescence being orange colored when manganese (Mn) is added as the luminescent center, and green colored when terbium trifluoride (TbF.sub.3) is added, for example.
In the thin-film EL display device 10 of the above structure, zinc sulfide with the addition of samarium trifluoride (SmF.sub.3), for example, has been considered for the material for forming a light-emitting layer 4 that exhibits red color luminescence.
However, the thin-film EL display device 10 using the light-emitting layer 4 formed from this material can only provide a luminous intensity as low as 1000 cd/cm.sup.2 at the maximum (when driven at 5 Khz) and has poor color purity since its emission spectrum contains components having wavelengths shorter than that of red light, and therefore, at the current level of development, it is not suitable for use in an EL panel or other display devices.
To overcome this problem, there has recently been proposed a method in which, in order to produce red colored light, a filter that cuts off the wavelengths of light shorter than 570 nm is used with a thin-film EL display device comprising a ZnS/Mn light-emitting layer that emits orange light. It is claimed that since the luminous intensity of the original orange light is high, this thin-film EL display device can produce red light of sufficient luminance even when passed through a filter.
However, since the filter is formed by a printing process using a paint containing a pigment and binder, its heat resisting temperature is as low as about 200.degree. C. It is therefore not possible to insert the filter during the thin-film EL display device fabrication process in which various layers are deposited on a glass substrate by vapor deposition, sputtering, etc. while the glass substrate is being heated. This leaves no other choice but to form the filter after formation of the various layers of the thin-film EL display device, which limits the selection of the position into which the filter can be inserted. There is also the problem that the paint characteristics suffer degradation by the heat generated during the light emitting operation of the thin-film EL display device, which not only causes the luminescent color to change with time but eventually leads to the deterioration of the device characteristics.
Furthermore, when depositing an insulating film of tantalum pentoxide (Ta.sub.2 O.sub.5) on the light-emitting layer made of zinc sulfide (ZnS) as the matrix, the surface of the zinc sulfide is oxidized by an oxygen plasma, resulting in the formation of a zinc sulfate (ZnSO.sub.4) layer. The formation of the zinc sulfate (ZnSO.sub.4) layer is dependent on such factors as the oxygen concentration, substrate temperature, and deposition time during the deposition of the insulating layer of tantalum pentoxide (Ta.sub.2 O.sub.5). Since zinc sulfate (ZnSO.sub.4) is extremely soluble in water, the problem is that the adhesion between the light-emitting layer and the insulating layer is impaired during a subsequent process such as a rinsing or cleaning process, giving a rise to the possibility of separation between these two layers.
Another problem with the prior art is that because of variations in the thickness of the zinc sulfate (ZnSO.sub.4) layer, etc., the light emitting characteristics and reliability of the thin film EL display device are extremely unstable.